Fix #11008: Support generic tuples as a valid unapply result

[anatoliykmetyuk]

On top #14296.

Done during the Issue Spree of 8th February together with @dwijnand and @prolativ

[dwijnand]

I think I overshot by thinking we should do a fold here. I think that's the cause of the failure. We should just traverse the type structure and do the various unwrapping calls ourselves, e.g dealiasing, unwrapping annotations, etc.

[anatoliykmetyuk]

It looks like the fold does the right thing here, the problem is that we didn't special-case for unapplySeq. I added a commit to do so, let's see if it breaks anything else.

[dwijnand]

Oh, actually it might not be the fold. But it's nothing to do with unapplySeq. We just didn't carry over the && !isUnapplySeq that was also present for the product branch.

Moving it has the same affect (though it keeps it separate from the product match). But that unapplySeq change isn't right.

[anatoliykmetyuk]

Why not? Shouldn't we also support generic tuples as a return type of unapplySeq, and shouldn't this be special-cased?

[dwijnand]

The point of unapplySeq is to define an extractor that can do runtime checks on the length of the sequence and operate on it (apply/drop/toSeq). But generic tuples, like all tuples and case class (i.e. all products), have a known length and we consume them as a products or product-like, not as a sequence, by directly accessing their components.

What use case were you thinking of?

[anatoliykmetyuk]

E.g.:

scala> object Foo:
     |   def unapplySeq(x: String): (Int, Seq[String]) = (x.length, x.toList.map(_.toString))

scala> "foo" match
     |   case Foo(1, c) => "One character " + c
     |   case Foo(x, xs*) => s"Many $x characters $xs"

We might want to use a generic tuple there.

[dwijnand]

Fair enough. But if you want to support it you should add tests that it actually works.

[bishabosha]

Note: there is no documentation that product pattern for generic tuples > 22 elements does not need _N accessors: https://docs.scala-lang.org/scala3/reference/changed-features/pattern-matching.html#product-match